Not sure it is relevant to this discussion. But let me chime in with a practical use cases coming from someone working for Digital Deluxe. They do, among other things, Japanese subtitling for American movies for the Japanese markets. The requirements from Japanese clients seem to be very strict with regards to the handling of Japanese subtitles (kanji, hiragana and katakana).
1. There should never be two movie characters having their subtitles at the same time on the screen.
2. It often happens when, for example, there is a building sign, or a text meaningful for understanding the movie, to put a vertical subtitles on the left or the right of the screen that would translate this sign. This can happen at the same time than the dialog subtitles.
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Karl Dubost - http://dev.opera.com/
Developer Relations & Tools, Opera Software